Festinger, schachter, and back (1950) traced friendship formations among couples in an apartment complex at mit. they found that residents were most likely to be friends with people

They found that residents were most likely to be friends with people who lived closer to them.

Explanation:

In their study, Festinger, Schachter, and Back required residents to identify their three closest friends in an apartment complex. As it was predicted by the propinquity effect, sixty-five percent of friends they named lived in the same building, no matter the distance to other buildings. And this was only one of many examples. In this way, these three researchers proved that propinquity is based both in physical and functional distance.

if im not mistaken its a. collective emotional arousal

the answer to your question is,

old stone age.

-mabel < 3